Sintered spherical pellets

ABSTRACT

Sintered, spherical composite pellets or particles comprising alumina fines, at least one of clay and bauxite and optionally a sintering aid, are described, along with a process for their manufacture. The use of such pellets in hydraulic fracturing of subterranean formations and in grinding is also described.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a non-provisional of U.S. patent applicationSer. No. 60/609,778, filed on Sep. 14, 2004, entitled “SinteredSpherical Pellets Useful for Gas and Oil Well Proppants,” which isincorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

The present invention relates to oil and gas well proppants and, moreparticularly, to proppants with excellent crush resistance in a broadrange of applications in which one of the starting raw materials used toproduce the proppant is alumina fines.

Oil and natural gas are produced from wells having porous and permeablesubterranean formations. The porosity of the formation permits theformation to store oil and gas, and the permeability of the formationpermits the oil or gas fluid to move through the formation. Permeabilityof the formation is essential to permit oil and gas to flow to alocation where it can be pumped from the well. Sometimes thepermeability of the formation holding the gas or oil is insufficient foreconomic recovery of oil and gas. In other cases, during operation ofthe well, the permeability of the formation drops to the extent thatfurther recovery becomes uneconomical. In such cases, it is necessary tofracture the formation and prop the fracture in an open condition bymeans of a proppant material or propping agent. Such fracturing isusually accomplished by hydraulic pressure, and the proppant material orpropping agent is a particulate material, such as sand, glass beads orceramic pellets, which are carried into the fracture by means of afluid.

Spherical particles of uniform size are generally acknowledged to be themost effective proppants due to maximized permeability. For this reason,assuming other properties to be equal, spherical or essentiallyspherical proppants, such as rounded sand grains, metallic shot, glassbeads, tabular alumina, or other ceramic raw materials mechanicallyprocessed into spheres are preferred.

DETAILED DESCRIPTION

In accord with the present invention, spherical pellets or particles,having alumina fines as one of the starting raw materials, are produced.The spherical particles are useful as oil and gas proppants as well asgrinding media. The spherical particles may be fired at a sinteringtemperature for a period of time sufficient to enable recovery ofsintered, spherical pellets having an apparent specific gravity ofbetween 2.70 and 3.75 and a bulk density of from about 1.35 to about2.15 g/cm³. The proppants of the present embodiments range fromintermediate to exceptionally high strength and are effective at closurestresses of up to about 15,000 psi. Thus, the proppants of the presentembodiments are generally applicable for moderate to very deep oil andgas wells where closure stresses may be extreme.

The spherical pellets are made from a composition of alumina fines, andat least one of clay and bauxite. In addition, sintering aids may beadded to the compositions. Suitable sintering aids include iron oxideand zinc oxide. The composition may include from about 10 to 75 percentby weight of alumina fines, from about 20 to 90 percent by weight of atleast one of clay and bauxite and, optionally, from about 0.1 to 15percent by weight of a sintering aid. The clay and bauxite may have analumina content ranging from about 35 to 90 percent by weight ofalumina. In one embodiment, the composition includes 58 percent byweight of alumina fines and 42 percent by weight of calcined kaolin. Inanother embodiment, the composition includes 64 percent by weight ofalumina fines, 28 percent by weight of kaolin and 8 percent by weight ofiron oxide. In yet another embodiment, the composition includes 20percent by weight of alumina fines and 80 percent by weight of bauxite.In a still further embodiment, the composition includes 36 percent byweight of alumina fines, 63 percent by weight of bauxite and 1 percentby weight of zinc oxide.

A suitable alumina fines material for use in the compositions forproducing the proppant of the present embodiments is the alumina finesdust collector by-product of alumina purification using the Bayerprocess. According to the Bayer process, the aluminum component ofbauxite ore is dissolved in sodium hydroxide, impurities are removedfrom the solution and alumina trihydrate is precipitated from thesolution and then calcined to aluminum oxide. A Bayer Process plant isessentially a device for heating and cooling a large recirculatingstream of caustic soda solution. Bauxite is added at the hightemperature point, red mud is separated at an intermediate temperature,and alumina is precipitated at the low temperature point in the cycle.The alumina fines that are useful for the preparation of the proppantpellets according to the present embodiments are a by-product thisprocess. A suitable alumina fines product has an alumina content ofabout 99 percent by weight, a loss on ignition of about 13% -22%, anaverage particle size of about 12 microns and about 86% or more of theparticle size distribution is less than 45 microns. The term “loss onignition” refers to a process, well known to those of ordinary skill inthe art, in which samples are dried at about 100° C. to drive off freemoisture and then heated to about 1000° C. to drive off chemically boundwater and other compounds.

The compositions for producing the proppant of the present embodimentsalso include at least one of clay and bauxite. A suitable clay materialfor use in the compositions for producing the proppant of the presentembodiments is kaolin. Kaolin as found in nature, is a hydratedaluminosilicate having a composition of approximately 52% SiO₂, and 45%Al₂O₃ (chemistry is in weight percent on a calcined basis). A suitablekaolin clay is mined in the McIntyre, Ga. area and has a loss onignition of approximately 14%. According to certain embodiments, thekaolin clay material may be calcined by methods well known to those ofordinary skill in the art, at temperatures and times to removesufficient water of hydration to facilitate pelletization.

A suitable bauxite material for use in the compositions for producingthe proppant of the present embodiments is available from Comalco. Thisbauxite as found in nature has a chemical composition of approximately82% Al₂O₃, 7% SiO₂ (chemistry is in weight percent on a calcined basis).The bauxite is mined and calcined in Australia and as received has aloss on ignition of approximately less than 1%. According to certainembodiments, the bauxite material may be calcined by methods well knownto those of ordinary skill in the art, at temperatures and times toremove sufficient water of hydration to facilitate pelletization.

The clay and bauxite materials for use in the compositions for producingthe proppant of the present embodiments are compatible with, and may beused as a matrix for, a wide variety of proppant materials, and, in thismanner, a wide variety of composite proppants may be produced, which maybe customized to particular conditions or formations. Thus, theproperties of the final sintered composite pellets, such as strength,permeability, apparent specific gravity, bulk density and acidresistance, may be controlled through variations in the initialcomponent mixture.

Further the spherical pellets may be customized for use as a grindingmedia. For instance, the desired color and density of the grinding mediacan be achieved by the appropriate selection of the starting materialsand in particular the specific sintering aid. A light colored media isoften a requirement for grinding media so that wear of the media duringgrinding will not discolor the product being milled. In this case, asintering aid such as iron oxide would not be used since iron oxidetends to darken the media. Instead, a sintering aid such as zinc oxidemay be used since it results in a light colored media. A desired densityof the grinding media can be achieved by adjusting the relative ratiosof the starting materials. For instance, ratios of starting ingredientsthat contain higher amounts of alumina fines or bauxites will result ina higher density media. Higher density grinding media improves millingin ball mills and vertical mills. The properties appropriate forgrinding media such as abrasion resistance are well known to those ofordinary skill in the art.

The term “apparent specific gravity,” as used herein, is a numberwithout units, but is defined to be numerically equal to the weight ingrams per cubic centimeter of volume, excluding void space or openporosity in determining the volume. The apparent specific gravity valuesgiven herein were determined by water displacement.

The term “bulk density”, as used herein, is defined to mean the weightper unit volume, including in the volume considered the void spacesbetween the particles.

Unless stated otherwise, all percentages, proportions and values withrespect to composition are expressed in terms of weight.

As noted above, the compositions for producing the proppant of thepresent embodiments may also include sintering aids such as iron oxideor zinc oxide. The iron oxide may be added to the composition ashematite iron oxide (Fe₂O₃) or other forms of iron oxide, such as FeOand Fe₃O₄, and thus the term “iron oxide” as used herein means all formsof iron oxide and may be generically represented as Fe_(x)O_(y). Asuitable iron oxide material is pigment grade iron oxide which iscommercially available from Densimix, Inc. A suitable zinc oxidematerial is commercially available from U.S. Zinc.

The present invention also provides a process for propping fractures inoil and gas wells at depths of up to 20,000 feet utilizing the proppantof the present embodiments by mixing the proppant with a fluid, such asoil or water, and introducing the mixture into a fracture in asubterranean formation. The compaction pressure upon the fracturegenerally is up to about 15,000 psi.

In a method of the present embodiments, the composition of aluminafines, at least one of clay and bauxite and optionally one or moresintering aids is ground into a fine particle size dust. This dustmixture is added to a high intensity mixer having a rotatable tableprovided with a rotatable impacting impeller, such as described in U.S.Pat. No. 3,690,622, to Brunner. Sufficient water is added to causeessentially spherical ceramic pellets to form. Optionally, a binder, forexample, various resins or waxes, starch, or polyvinyl alcohol known inthe prior art, may be added to the initial mixture to improvepelletizing and to increase the green strength of the unsinteredpellets. A suitable binder is starch which may be added at levels offrom about 0 to 1.5 percent by weight. In certain embodiments, thestarch may be added at an amount of from about 0.5 to 0.7 percent byweight.

The resulting pellets are then dried and screened to an appropriatepre-sintering size, and fired at sintering temperature until an apparentspecific gravity between about 2.70 and about 3.75 is obtained,depending on the composition of the starting mixture.

Those of ordinary skill in the art will recognize that the compositionmay also include other conventional sintering aids including, forexample, minor amounts of bentonite clay, feldspar, nepheline syenite,talc, titanium oxide, and compounds of lithium, sodium, magnesium,potassium, calcium, manganese and boron, such as lithium carbonate,sodium oxide, sodium carbonate, sodium silicates, magnesium oxide,magnesium carbonate, calcium oxide, calcium carbonate, manganese oxide,boric acid, boron carbide, aluminum diboride, boron nitride and boronphosphide. The most desirable range of sintering aid can be readilydetermined by those skilled in the art, depending upon the particularmixture of alumina fines, clay and bauxite used.

The sintered proppant pellets of the present embodiments are sphericalin shape. The sphericity of the proppant pellets was determined using avisual comparator. Krumbein and Sloss, Stratigraphy and Sedimentation,second edition, 1955, W. H. Freeman & Co., San Francisco, Calif.,describe a chart for use in visual determination of sphericity androundness. Visual comparison using this chart is a widely used method ofevaluating sphericity or roundness of particles. In using the visualcomparison method, a random sample of 20 particles of the material to betested is selected. The particles are viewed under a 10 to 20 powermicroscope or a photomicrograph and their shapes compared to theKrumbein and Sloss chart. The chart values for sphericity range from 0.3to 0.9. The chart values for the individual particles are then averagedto obtain a sphericity value.

The term “spherical” and related forms, as used herein, is defined tomean an average ratio of minimum diameter to maximum diameter of about0.80 or greater, or having an average sphericity value of about 0.8 orgreater compared to a Krumbein and Sloss chart. The sintered proppantpellets of the present embodiment have an average sphericity of about0.8 or greater when visually compared with the Krumbein and Sloss chart.The proppant pellets of certain of the present embodiments have aroundness of about 0.9 and a sphericity of about 0.9.

A suitable procedure for producing the sintered, spherical pellets ofthe present embodiment is as follows:

1. The starting ingredients of alumina fines, one or both of clay andbauxite, optionally one or more sintering aids, and optionally binderare ground to about 90-100% less than 325 mesh. According to certainembodiments, one or both of the clay and bauxite is calcined, and ifpresent, the binder is starch. Ninety weight percent of the groundstarting ingredients are added to a high intensity mixer.

2. The starting ingredients are stirred using a suitable commerciallyavailable stirring or mixing devices have a rotatable horizontal orinclined circular table and a rotatable impacting impeller.

3. While the mixture is being stirred, sufficient water is added tocause formation of spherical pellets and growth of those pellets to thedesired size.

In general, the total quantity of water which is sufficient to causeessentially spherical pellets to form is from about 17 to about 23percent by weight of the starting ingredients. The total mixing timeusually is from about 2 to about 15 minutes.

After the mixture of alumina fines, at least one of clay and bauxite,optionally one or more sintering aids and optionally binder has growninto spherical pellets of the desired size, the mixer speed is reduced,and 10 weight percent of the ground starting ingredients is added to themixer.

4. The resulting pellets are dried and screened to a specified size thatwill compensate for the shrinkage that occurs during sintering in thekiln. The pellets are screened for size preferably after drying. Therejected oversized and undersized pellets and powdered material obtainedafter the drying and screening steps may be recycled.

5. The dried pellets are then fired at sintering temperature for aperiod sufficient to enable recovery of sintered, spherical pelletshaving an apparent specific gravity of between 2.70 and 3.75 and a bulkdensity of from about 1.35 to about 2.15 g/cm³. The specific time andtemperature to be employed is dependent on the starting ingredients andis determined empirically according to the results of physical testingof pellets after firing.

Pellets may also be screened after firing. The finished pellets may betumbled to enhance smoothness. The proppant of the present embodimentsgenerally has a particle size distribution that meets the APIdesignation for 20/40 proppant which specifies that the product mustretain 90% between the primary 20 and 40 mesh sieves. However, othersizes of proppant ranging from 140 mesh to 6 mesh may be produced withthe same mixture. The proppant prepared according to the presentembodiments demonstrates the following typical sieve analysis (weight %retained): U.S. Mesh Microns 20/40 +16 +1180 0 −16 + 20 −1000 + 850  3−20 + 30 −850 + 600 69 −30 + 40 −600 + 425 27 −40  −425 0

The bulk density values reported in Table I were determined by weighingthat amount of sample that would fill a cup of known volume utilizingprocedure ANSI B74.4.

The crush values reported in Table I were obtained using the AmericanPetroleum Institute (API) procedure for determining resistance tocrushing. According to this procedure, a bed of about 6 mm depth ofsample to be tested is placed in a hollow cylindrical cell. A piston isinserted in the cell. Thereafter, a load is applied to the sample viathe piston. One minute is taken to reach maximum load which is then heldfor two minutes. The load is thereafter removed, the sample removed fromthe cell, and screened to separate crushed material. The results arereported as a percentage crushed to a size smaller than the startingmaterial by weight of the original sample (e.g. for a 20/40 material itwould be the material that was crushed to −40 mesh).

In Table I is summarized the composition of the present embodiments forpellets produced from the raw materials indicated. Also given are theresults of testing of these pellets. All samples were prepared in accordwith the procedures described herein. Examples 1-4 give detailsregarding the procedure employed in the preparation of the proppantsamples the testing of which is reported in Table I.

The chemistries for the mixtures were calculated from the blendingratios of raw materials and the chemistries of the raw materials asmeasured by inductively coupled plasma (ICP) which is an analyticalmethod known to those of ordinary skill in the art. TABLE I Alumina/Alumina/Kaolin/ Alumina/Bauxite/ Alumina Kaolin Iron Oxide ZnOAlumina/Bauxite Fines (58:42) (64:28:8) (36:63:1) (20:80) ChemistryAl₂O₃ 98.77 77.18 77.96 89.8 85.6 Fe₂O₃ 0.03 0.43 7.95 0.7 5.4 K₂O 00.04 0.02 0.1 0.01 SiO₂ 0.08 21.31 12.57 4.9 5.7 CaO 0.04 0.1 0.09 0.20.02 NaO 1.07 N/A 0.74 0.4 0.2 MgO 0 0.04 0.02 0.1 0.02 P₂O₅ 0 0.04 0.010.1 0.01 TiO₂ 0 0.86 0.57 2.4 2.9 ZnO 0 0 0 1.0 0 LOI 15.6 13.4 12.414.9 4.2 20/40 Properties BD 1.34 1.84 1.99 1.98 ASG N/A 3.3 3.65 3.6315,000 psi % crush 7.4 2.9 10,000 psi % crush 2.9 4,000 psi % crush 2.1Sphericity >0.8 >0.8 >0.8 >0.8

EXAMPLE 1

A 58/42 ratio mixture of alumina fines and calcined kaolin clay wasprepared by first grinding the mixture so that 99.4% of the mixture hada particle size of less than 325 mesh. Next, about 3200 grams of the58/42 ratio mixture was charged to an R02 Eirich mixer.

The mixer was operated on high speed rotor and 1050 grams of watercontaining 24 grams of starch as a binding agent was added. Pelletizingwas continued at high speed rotor for 4.5 minutes. Next, the speed ofthe mixer was reduced to “slow” rotor and 200 grams of polishing dusthaving the same 58/42 ratio composition of alumina fines and calcinedkaolin clay was added. The pellets were polished under slow rotor for atotal of 1.5 minutes.

The pellets were then dried and screened to −16 mesh/+30 mesh prior tofiring at temperatures ranging from 2850 to 3000° F. The resultingpellets had a bulk density of 1.34 gm/cm³.

The crush strength of the pellets was tested in accordance with the APIprocedure for determining resistance to crushing noted above and at aninduced pressure of 4,000 psi the pellets had a crush percentage of 2.1.The best strength was obtained at firing temperatures of 3000° F. whichwas the maximum temperature capability of the laboratory furnace. Thedata indicate that firing the pellets from this blend at highertemperatures would generate pellets with optimum strength.

EXAMPLE 2

About 3200 grams of a 64/28/8 ratio mixture of alumina fines, calcinedkaolin clay, and iron oxide having a particle size of 98.6%<325 meshwere added to an R02 Eirich mixer.

The mixer was operated on high speed rotor and 750 grams of watercontaining 24 grams starch binder which is commercially available underthe trade name Staramic 100 from Tate and Lyle North America was added.Rotation of the table and impeller was continued for about 10.5 minutes;subsequently, the impeller speed was decreased and 200 grams ofpolishing dust having the same 64/28/8 ratio composition of aluminafines, calcined kaolin clay and iron oxide was added incrementally.Polishing continued for approximately 2 minutes.

The pellets were then dried and screened to −16 mesh/+30 mesh prior tofiring at about 2,750° F. The resulting pellets had an apparent specificgravity of about 3.30, a bulk density of 1.84 gm/cm³ and a sphericity ofgreater than 0.8, as determined using the Krumbein and Sloss chart.

The crush strength of the pellets was tested in accordance with the APIprocedure for determining resistance to crushing noted above and at aninduced pressure of 10,000 psi the pellets had a crush percentage of 2.9which meets the API specification of 10% maximum crush for this sizeproppant.

EXAMPLE 3

About 4.5 kilograms of a 36/63/1 ratio mixture of alumina fines,bauxite, and zinc oxide having a particle size of 99.9%<325 mesh wereadded to an R02 Eirich mixer.

The mixer was operated on high rotor speed and about 1000 grams of waterwas added. Rotation of the table and impeller was continued for about 6minutes; subsequently, the impeller speed was decreased and about 450grams of polishing dust having the same 36/63/1 ratio composition ofalumina fines, bauxite, and zinc oxide was added incrementally.Polishing continued for approximately 1 minute.

The pellets were then dried and screened to −16 mesh/+30 mesh prior tofiring at about 2,840° F. The resulting pellets had an apparent specificgravity of about 3.65, a bulk density of 1.99 gm/cm3 and a sphericity ofgreater than 0.8, as determined using the Krumbein and Sloss chart.

The crush strength of the pellets was tested in accordance with the APIprocedure for determining resistance to crushing noted above and at aninduced pressure of 15,000 psi the pellets had a crush percentage of 7.4which meets the API specification of 10% maximum crush for this sizeproppant.

EXAMPLE 4

About 3.6 kilograms of a 20/80 ratio mixture of alumina fines andbauxite having a particle size of 99.9%<325 mesh were added to an R02Eirich mixer.

The mixer was operated on high rotor speed and about 800 grams of waterwas added. Rotation of the table and impeller was continued for about 6minutes; subsequently, the impeller speed was decreased and about 360grams of polishing dust having the same 20/80 ratio composition ofalumina fines and bauxite was added incrementally. Polishing continuedfor approximately 1 minute.

The pellets were then dried and screened to −16 mesh/+30 mesh prior tofiring at about 2,750° F. The resulting pellets had an apparent specificgravity of about 3.63, a bulk density of 1.98 gm/cm3 and a sphericity ofgreater than 0.8, as determined using the Krumbein and Sloss chart.

The crush strength of the pellets was tested in accordance with the APIprocedure for determining resistance to crushing noted above and at aninduced pressure of 15,000 psi the pellets had a crush percentage of 2.9which meets the API specification of 10% maximum crush for this sizeproppant.

The spherical, sintered pellets of the present invention are useful as apropping agent in methods of fracturing subterranean formations toincrease the permeability thereof, particularly those formations havinga compaction pressure of up to about 15,000 psi, which are typicallylocated at depths of up to about 20,000 feet.

When used as a propping agent, the pellets of the present invention maybe handled in the same manner as other propping agents. The pellets maybe delivered to the well site in bags or in bulk form along with theother materials used in fracturing treatment. Conventional equipment andtechniques may be used to place the spherical pellets as propping agent.

A viscous fluid, frequently referred to as “pad”, is injected into thewell at a rate and pressure to initiate and propagate a fracture in thesubterranean formation. The fracturing fluid may be an oil base, waterbase, acid, emulsion, foam, or any other fluid. Injection of thefracturing fluid is continued until a fracture of sufficient geometry isobtained to permit placement of the propping pellets. Thereafter,pellets as hereinbefore described are placed in the fracture byinjecting into the fracture a fluid into which the pellets havepreviously been introduced and suspended. The propping distribution isusually, but not necessarily, a multi-layer pack. Following placement ofthe pellets, the well is shut-in for a time sufficient to permit thepressure in the fracture to bleed off into the formation. This causesthe fracture to close and apply pressure on the propping pellets whichresist further closure of the fracture.

In addition, the spherical, sintered pellets of the present inventionare useful as grinding media. When used as grinding media, the pelletsare nearly white or pale tan in color, a desirable property for mediaused in mineral grinding or other types of grinding where color of theground product is a critical quality parameter. When the spherical,sintered pellets of the present invention eventually wear during use,they do not produce discoloration in the product as is found with metalmedia or dark-colored ceramic media.

The foregoing description and embodiments are intended to illustrate theinvention without limiting it thereby. It will be understood thatvarious modifications can be made in the invention without departingfrom the spirit or scope thereof.

1. A gas and oil well proppant comprising a plurality of sintered,spherical pellets, said pellets being prepared from a compositioncomprising alumina fines and at least one of clay and bauxite.
 2. Theproppant of claim 1 wherein the composition further comprises at leastone sintering aid.
 3. The proppant of claim 2, wherein the at least onesintering aid is selected from the group consisting of iron oxide, zincoxide, bentonite clay, feldspar, nepheline syenite, talc, titaniumoxide, lithium carbonate, sodium oxide, sodium carbonate, sodiumsilicates, magnesium oxide, magnesium carbonate, calcium oxide, calciumcarbonate, manganese oxide, boric acid, boron carbide, aluminumdiboride, boron nitride and boron phosphide.
 4. The proppant of claim 2,wherein the composition comprises from about 10 to about 75 percent byweight of alumina fines, from about 20 to about 90 percent by weight ofat least one of clay and bauxite, and from about 0.1 to about 15 percentby weight of the at least one sintering aid.
 5. The proppant of claim 4,wherein the composition comprises from about 40 to about 75 percent byweight of alumina fines, from about 20 to about 60 percent by weightclay and from about 0.1 to about 15 percent by weight of at least onesintering aid.
 6. The proppant of claim 5, wherein the compositioncomprises kaolin clay.
 7. The proppant of claim 6, wherein thecomposition comprises calcined kaolin clay.
 8. The proppant of claim 4,wherein the composition comprises from about 10 to about 75 percent byweight of alumina fines, from about 20 to about 90 percent by weight ofbauxite, and from about 0.1 to about 15 percent by weight of the atleast one sintering aid.
 9. The proppant of claim 1, wherein thecomposition comprises 58 percent by weight of alumina fines and 42percent by weight of clay.
 10. The proppant of claim 2, wherein thecomposition comprises 64 percent by weight of alumina fines, 28 percentby weight of clay and 8 percent by weight of the at least one sinteringaid.
 11. The proppant of claim 10, wherein the at least one sinteringaid comprises iron oxide.
 12. The proppant of claim 2, wherein thecomposition comprises 36 percent by weight of alumina fines, 63 percentby weight of bauxite and 1 percent by weight of the at least onesintering aid.
 13. The proppant of claim 12, wherein the at least onesintering aid comprises zinc oxide.
 14. The proppant of claim 1, whereinthe composition comprises 20 percent by weight of alumina fines and 80percent by weight of bauxite.
 15. The proppant of claim 1, wherein thecomposition comprises kaolin clay.
 16. The proppant of claim 15, whereinthe composition comprises calcined kaolin clay.
 17. The proppant ofclaim 1, wherein the pellets have an apparent specific gravity of fromabout 2.70 to about 3.75.
 18. The proppant of claim 1, wherein thepellets have a bulk density of from about 1.35 to about 2.15 g/cm³. 19.A method of fracturing a subterranean formation located at a depth of upto about 20,000 feet, comprising: injecting a hydraulic fluid into theformation at a rate and pressure sufficient to open a fracture therein,and injecting into the fracture a fluid containing sintered, sphericalpellets, the pellets being prepared from a composition comprisingalumina fines and at least one of clay and bauxite.
 20. The method ofclaim 19 wherein the composition further comprises at least onesintering aid.
 21. The method of claim 20, wherein the at least onesintering aid is selected from the group consisting of iron oxide, zincoxide, bentonite clay, feldspar, nepheline syenite, talc, titaniumoxide, lithium carbonate, sodium oxide, sodium carbonate, sodiumsilicates, magnesium oxide, magnesium carbonate, calcium oxide, calciumcarbonate, manganese oxide, boric acid, boron carbide, aluminumdiboride, boron nitride and boron phosphide
 22. The method of claim 20,wherein the composition comprises from about 10 to about 75 percent byweight of alumina fines, from about 20 to about 90 percent by weight ofat least one of clay and bauxite, and from about 0.1 to about 15 percentby weight of the at least one sintering aid.
 23. The method of claim 22,wherein the composition comprises from about 40 to about 75 percent byweight of alumina fines, from about 20 to about 60 percent by weightclay and from about 0.1 to about 15 percent by weight of at least onesintering aid.
 24. The method of claim 23, wherein the compositioncomprises kaolin clay.
 25. The method of claim 24, wherein thecomposition comprises calcined kaolin clay.
 26. The method of claim 22,wherein the composition comprises from about 10 to about 75 percent byweight of alumina fines, from about 20 to about 90 percent by weight ofbauxite, and from about 0.1 to about 15 percent by weight of the atleast one sintering aid.
 27. The method of claim 19, wherein thecomposition comprises 58 percent by weight of alumina fines and 42percent by weight of clay.
 28. The method of claim 20, wherein thecomposition comprises 64 percent by weight of alumina fines, 28 percentby weight of clay and 8 percent by weight of the at least one sinteringaid.
 29. The method of claim 28, wherein the at least one sintering aidcomprises iron oxide.
 30. The method of claim 20, wherein thecomposition comprises 36 percent by weight of alumina fines, 63 percentby weight of bauxite and 1 percent by weight of the at least onesintering aid.
 31. The method of claim 30, wherein the at least onesintering aid comprises zinc oxide.
 32. The method of claim 19, whereinthe composition comprises 20 percent by weight of alumina fines and 80percent by weight of bauxite.
 33. The method of claim 19, wherein thecomposition comprises kaolin clay.
 34. The method of claim 33, whereinthe composition comprises calcined kaolin clay.
 35. The method of claim19, wherein the pellets have an apparent specific gravity of from about2.70 to about 3.75.
 36. The method of claim 19, wherein the pellets havea bulk density of from about 1.35 to about 2.15 g/cm³.